The present invention relates to an image forming apparatus applicable to color printers, color copiers and color facsimiles, and more particularly, to a belt unit and a color image forming apparatus having an intermediate transfer belt for superimposing toner images of a plurality of colors and a cleaner for removing residual toner on the intermediate transfer belt.
As an example of conventional color image forming apparatuses, a color electrophotographic apparatus is described in Japanese Laid-open Patent Application No. Hei 8-286455. FIG. 13 is a cross-sectional elevation view electrophotographic apparatus.
In FIG. 13, a belt unit 201 includes a transfer belt 202, a first transfer roller 203, a second transfer roller 204, a backup roller 205 and a guide roller 206. Color images are superimposed on the transfer belt 202 of the belt unit 201. In the left part of FIG. 13, an image forming unit 207 is disposed in which four fan-shaped image forming units 207Bk, 207Y, 207M and 207C for black, yellow, magenta and cyan, respectively, are arranged so as to form a circle. A laser exposure unit 212 is disposed in an upper part of the printer.
By attaching the image forming unit 207 in the printer, a mechanical drive system and an electric circuit system on the printer side are coupled to the image forming unit 207 by a non-illustrated inter-coupling member, so that the image forming unit 207 is mechanically and electrically connected to the printer. The image forming units 207Bk, 207Y, 207M and 207C disposed on a circle are rotated around a fixed mirror 213 by a driving motor (not shown). At the time of image formation, the image forming units 207Bk to 207C are rotated, and successively come to an image formation position 210 opposed to the first transfer roller 203 which supports the intermediate transfer belt 202. The image formation position 210 is also the position of exposure, whereat a photoconductor drum 218 is exposed by a laser beam 211.
The laser beam 211 is made incident through the gap between the image forming units 207Bk and 207C, and reflected at the mirror 213. The reflected laser beam 211 is made incident on an exposure section on the left side surface of the photoconductor drum 218 of the black image forming unit 207Bk, which is situated at the image formation position 210. The laser beam 211 scans in the direction of the generator of the photoconductor drum 218 which direction is vertical to the plane of FIG. 13 and exposes the photoconductor drum 218, so that a latent image is formed. The latent image is developed by a developer unit 219. The toner image developed by the developer unit 219 is transferred onto the transfer belt 202. Then, the image forming unit 207 rotates 90 degrees in the direction of an arrow R, so that the yellow image forming unit 207Y is situated at the image formation position 210 instead of the image forming unit 207Bk. Then, the same operation as that of the above-mentioned black image formation is performed, and the yellow image is formed in a superimposed manner on the black toner image having already been formed on the intermediate transfer belt 202. Similar operations are successively performed by use of the magenta and cyan image forming units 207M and 207C, so that a full color image is completed on the intermediate transfer belt 202. Then, one sheet of recording paper picked up from a paper feed unit (not shown) is conveyed, being timed by a resist roller 214, to a nip part where the intermediate transfer belt 202 and a secondary transfer roller 215 are in contact. The toner images of the four colors superimposed on one another are transferred onto the recording paper at a time, and fixed by a fuser unit 216. The recording paper having undergone fusing is discharged toward the right side of the figure. The toner remaining on the intermediate transfer belt 202 is removed by a cleaner roller 217. To the cleaner roller 217, a positive voltage is applied which is opposite in polarity to the charge applied to the toner. During toner image formation, in order to prevent toner from adhering to the cleaner roller 217, a voltage of a polarity (negative) the same as that of the charge applied to the toner is applied to the cleaner roller 217 which is always pressed against the intermediate transfer belt 202.
Another conventional example, Japanese Laid-open Patent Application No. Sho 57-169781 discloses a cleaner shown in FIG. 14. In FIG. 14, a rigid cleaner backup member 220 held by a spring 224 and a cleaner roller 225 pinch a transfer belt 223 entrained about support rollers 221 and 222 for conveying recording paper. The cleaner backup member 220 is formed so as to be always biased toward the cleaner roller 225 by the spring 224. FIG. 15 is an enlarged view showing the part where the cleaner roller 225 and the transfer belt 223 are in contact.
As still another conventional example, as shown in FIG. 16, there is known structure having a cleaner roller 230 energized with a voltage of inverse polarity to the charge of the toner. The cleaner roller 230 is operated so as to be alternatingly separated from and brought into contact with a semi-conductive intermediate transfer belt 233 which is entrained between support rollers 231 and 232 which are connected to GND potential.
Further, in a cleaner 244 of a transfer belt according to another conventional example shown in the cross-sectional view of FIG. 17, a blade 240 of urethane is pressed against a transfer belt 241, and toner is scraped off by the blade 240. The scraped toner is scooped by a scoop seal 242 so as not to spill through the gap between the transfer belt and a toner case 243, and is collected into the toner case 243. This is known as a cleaning blade method.
Color image forming apparatuses are required not only to improve the throughput and the positioning accuracy for accurately positioning the images of a plurality of colors but also to improve cleaning capability for completely removing unnecessary toner on the intermediate transfer belt.
In the conventional example of FIG. 13, the cleaner roller 217 is always pressed against the intermediate transfer belt 202, and switching between removal and non-removal of toner is made by changing the polarity of the voltage. Even if the toner on the intermediate transfer belt 202 is prevented from being removed by applying to the cleaner roller 217 a voltage which is the same in polarity as the charge applied to the toner, there are cases where toner is removed by a frictional force other than the electric force, and the toner image is liable to be disturbed.
To avoid the above-mentioned defect, a structure in which the cleaner is separated from and brought into contact with the intermediate transfer belt has been put to practical use. In this structure, when cleaning is performed by bringing the cleaner in contact with the intermediate transfer belt after the formation of a color image on the intermediate transfer belt by superimposing toner images of four colors is completed, the length of the belt from the transfer position to the cleaner position must be longer than the length of the image range. Consequently, the circumferential length of the belt increases, so that the belt unit and the body of the apparatus increase in size. In addition, the time necessary for the belt to rotate once increases, so that the throughput decreases.
In normal color image forming apparatuses, the images of four colors are put together in position and superimposed with each other on the intermediate transfer belt. Therefore, high positioning accuracy is required in order to prevent displacement of positions of the images of respective colors. The positional displacement among the images of the respective colors will hereinafter be referred to as mere "positional displacement."
The accuracies of the rollers about which the intermediate transfer belt 202 is entrained largely affect the positioning accuracy. For this reason, in the conventional example of FIG. 13, the backup roller 205 about which the intermediate transfer belt 202 is entrained is required to have considerable rigidity.
The separation from and contact with the backup roller 205 of the cleaner roller 217 largely vary the load on the intermediate transfer belt 202. When the load acting on the intermediate transfer belt 202 varies due to the separation and contact of the cleaner roller 217, the amount of slip that is steadily and slightly caused between the drive shaft and the intermediate transfer belt 202 varies. Consequently, the transportation speed of the intermediate transfer belt 202 varies between the case where the cleaner roller 217 is separated and the case where the cleaner roller 217 is in contact, so that a positional displacement is caused among the images of the colors on the intermediate transfer belt 202. The greater the load variation is, the larger speed variation due to the separation and contact of the cleaner roller 217 is, and the more conspicuous the positional displacement is. In addition, the load variation due to the separation and contact of the cleaner roller 217 deforms the drive member of the intermediate transfer belt 202. Consequently, a positional displacement is caused between the toner image formed when the cleaner roller 217 is separated and the toner image formed when the roller 217 is in contact.
In the conventional art of FIG. 13, since the belt unit 201 has a low-profile configuration being elongated in the horizontal direction of the figure, the distance between the primary transfer section and the secondary transfer section is long, so that the horizontal length of the body of the apparatus is large. Moreover, the part of the cleaner roller 217 disposed outside the intermediate transfer belt 202 protrudes in the upper right direction of the figure. Further, in order to attach and detach the intermediate transfer belt 201 to and from the body of the apparatus in a slanting direction from the upper right, the opening through which the intermediate transfer belt 201 passes when it is attached and detached is necessarily large. For this reason, a front door 227 for opening and closing the opening includes the fuser unit 216, so that the size and the weight increase.
There is a space on the left of a paper feed mechanism such as the resist roller 214 and below the belt unit 201. Thus, the space in the apparatus is not effectively used and the size of the apparatus increases accordingly. Further, since recording paper whereon fusing is over is discharged toward the right side of the figure (front side of the apparatus), the paper discharge tray (not shown) protrudes rightward, so that the floor area of possession increases.
In the structure of the prior art shown in FIG. 14 in which the rigid cleaner backup member 220 is pressed against the cleaner roller 225 with elasticity, as shown in FIG. 15, the backup member 220 is in contact with the transfer belt 223 only in a range B which is a part of a nip part S of the cleaner roller 225. For this reason, the backup member 220 as an opposed electrode contacts only a part of the nip part S. Consequently, the potential of the semi-conductive transfer belt 223 is unstable at the contact part of the nip part S because of potential variation of the adjoining belt support shaft and movement of charges from the cleaner roller 225. This weakens the force of the electric field that moves toner from the transfer belt 223 to the cleaner roller 225, so that the cleaning capability is deteriorated.
In the prior art shown in FIG. 14 and FIG. 15, W represents the region of encroachment of the cleaner roller 225 on a common tangential line P of the rollers 221 and 222 about which the transfer belt 223 is entrained. The backup member 220 cannot push the transfer belt 223 against the cleaner roller 225 in the entire encroachment region W. The transfer belt 223 and the cleaner roller 225 contact only in the range of the nip part S in the vicinity of the central part. Since the nip part S is narrow, the contact between the cleaner roller 225 and the transfer belt 223 is unstable because of depressions or deformation of the surface of the cleaner roller 225. Consequently, the frictional force and the electric field force affecting to the toner on the transfer belt 223 weaken, so that the cleaning capability is deteriorated.
In the prior art shown in FIG. 16, the cleaner roller 230 is energized with a voltage of inverse polarity to the charge of the toner. The cleaner roller 230 is operated so as to be alternatingly separated from and brought into contact with the semi-conductive intermediate transfer belt 233 which is entrained between the support rollers 231 and 232. Since an opposed electrode like the backup member 220 in FIG. 15 is absent, the potential of the semi-conductive intermediate transfer belt 233 is unstable in the nip part because of movement of charges from the cleaner roller 230 and other high-potential members. Consequently, the electric field force that moves toner from the intermediate transfer belt 233 to the cleaner roller 230 weakens, so that the cleaning capability is deteriorated. When the potential of the adjoining roller is not the GND potential, the potential of the intermediate transfer belt 233 at the cleaning position is particularly unstable because it is dependent on the electrical resistance and the thickness of the intermediate transfer belt 233.
In the cleaning blade method of the prior art shown in FIG. 17, when the blade 240 is separated from the intermediate transfer belt 241, the inside of the toner case 243 communicates with the outside between an end of the blade 240 and the scoop seal 242. At this time, the toner collected into the toner case 243 by cleaning spills and contaminates the inside of the apparatus. Further, when only a cleaner 244 is taken out of the apparatus with the toner case 243 being filled with toner, toner spills inside and outside the apparatus.
Thus, in the cleaning blade method, if the cleaning surface of the intermediate transfer belt 241 inclines upward from the vertical even slightly, when the blade 240 is separated from the intermediate transfer belt 241, the toner that heaps up at the end of the blade 240 drops onto the intermediate transfer belt 241, so that the intermediate transfer belt 241 and the inside of the apparatus are contaminated. For this reason, an upward-facing surface cannot be used as the cleaning surface.
A conventional belt unit drive mechanism is structured so that when the belt unit is attached, the drive roller for rotating the intermediate transfer belt and the drive system on the body side relatively move in the direction of axis of the drive roller and engage with each other. In this structure, for example, it is necessary to insert and take out a drive coupler after the belt unit is attached, and a mechanism therefor is necessary. In a structure in which a drive gear is fixed to the drive roller and disposed in the rear of the belt unit in the attachment direction, it is necessary to provide in the belt unit a relief for avoiding interference with the body-side gear when the belt unit is attached. Because of this, the volume of possession of the belt unit cannot be effectively used, so that the belt unit increases in size and the body of the apparatus also increases in size.
In a conventional belt unit positioning mechanism, a positioning mechanism is disposed in the rear of the belt unit in the attachment direction. In this structure, the attitude of the belt unit on the front side in the attachment direction is unstable, so that it is difficult to insert the belt unit in the normal position when it is attached. When the positioning mechanism and the drive gear are separated from each other, it is difficult to ensure the position accuracy of the body-side gear and the drive gear of the belt unit.
When the pressure is too high at the nip part of the secondary transfer section, the toner inside the edge part of the image is not transferred to recording paper, that is, an inner part of the image is missing. To prevent this, the secondary transfer roller is pressed against the intermediate transfer belt at a constant pressure.
In the above-mentioned conventional structure, the recording paper conveyance path is bent at the secondary transfer position, so that the angle of the recording paper on the side of the secondary transfer roller 215 is smaller than 180 degrees. Therefore, when the secondary transfer roller 215 is pressed against the intermediate transfer belt 202 at a constant pressure, the pressure at the nip part of the secondary transfer section varies due to variation in tension of the recording paper, so that nonuniformity in transfer is caused.
When the recording paper conveyance direction is not vertical at the secondary transfer position, the paper conveyance path is complicated, so that the number of elements such as a paper guide increases. This increases the weight of the front door 227 being opened for paper processing such as removal of jammed paper. As a result, the size of the apparatus increases.
When a tension roller is used, the larger the angle at which the recording paper is turned along the tension roller is, the higher the pressure applied to the tension roller is. When the pressure is high, the frictional force at a moving part of the tension roller is large. This hinders smooth movement, so that the intermediate transfer belt is hindered from being transported with stability.
In order to prevent the positional displacement due to variation in circumferential speeds of the photoconductor drum 218 and the intermediate transfer belt 202, it is necessary to synchronize the rotations of the photoconductor drum 218 and the intermediate transfer belt 202. To do so, a structure is used in which the rotations are synchronized by setting the rotation ratio between the intermediate transfer belt 202 and a drive roller to an integer, and the photoconductor drum 218 and a belt drive shaft are rotated by a series of drive system. Although the drive roller is not specified in the above-mentioned conventional example, when the drive roller is situated away from the photoconductor onto which the primary transfer is performed, it is difficult to form the drive shaft of the photoconductor drum 218 and the belt drive shaft as a series of drive system.